In certain computer control applications, it is necessary to track and measure the image of an object passively. It is especially important in weapons delivery systems that a target be so tracked. If such a target were tracked actively (i.e., using radar or laser range finding techniques), the target might detect the presence of the tracker. Once the target has detected the presence of the tracker, it can respond in one of several ways, all of which are deleterious to the tracker. For instance, the target might "jam" the tracker by bombarding it with signals that are comparable to those which the tracker is actively using or the target might fire its own weapon at the tracker, at the source of the tracking signal, or, even at the launching site of the tracker. In this way, the target could defeat the tracker, destroy the tracker or perhaps even destroy the launch site of the tracker, including the operating personnel.
Passively tracking a target, however, imposes at least one serious limitation on the tracker. A tracker cannot accurately determine the distance or "range" to a target if it cannot actively sense the object. An active tracker, for instance, could determine the distance to a target by measuring the elapsed time from the emission of a radio frequency signal to the receipt of the signal reflected off of the target. The absence of a range measurement from tracker to target limits the passive tracker's ability to compensate for the apparent change in target image as the tracker moves in relationship to the target. Without this ability, a tracker will fail to maintain a constant target aimpoint.
In practice, a tracker benefits by tracking several subimages of its target's image. These subimages are two dimensional representations of structures that are physically related to the exact target location or "aimpoint" in the real three-dimensional world. Multiple subimages are used for redundancy purposes and because the actual aimpoint of the target is often untrackable due to low image contrast, brightness, or other reasons. As the tracker nears the target, however, the subimages will appear to radiate outwardly with respect to each other. The position of the subimages with respect to one another may also change in other ways in certain situations. For instance, two subimages located on a target may appear to approach one another if they are located on a face of a target that is rotating away from the tracker. A subimage associated with a straight edged structure or a line poses other problems for a passive tracker. Such a subimage is indistinguishable from other subimages corresponding to other points on the line. This ambiguity can introduce serious error into a track. Straight edges typically appear when tracking structures such as buildings, bridges, dams or internal detail on armored vehicles. The tracker motion can be further complicated by background subimages erroneously tracked by the tracker. A tracker will then mislocate the aimpoint and, perhaps, completely miss its target if it cannot identify and compensate for "bad" subimages.
Prior attempts to passively track an object have resulted in solutions with limited flexibility and poor accuracy. Heretofore, an object once identified as an aimpoint was tracked by tracking a predetermined number of subimages in a known pattern. Typically, the pattern chosen was a square with the aimpoint at its center and four subimages located at the four corners of the square. That system would track the four subimages located at the corners of the square and infer the actual aimpoint using the simple symmetry of the predetermined square. This method faltered when the geometry of the actual target resulted in less than four suitable subimages located in the requisite pattern or when the subimage selected was not associated with the aimpoint. This system lacked the ability to identify bad subimages.
Therefore, a need has arisen for a passive subimage tracker which is able to identify trackable subimages in arbitrary image locations and use them to track an aimpoint.